The present invention relates to a toroidal continuously variable transmission (CVT) for a vehicle, and more specifically to a power roller support structure for the toroidal CVT.
In general, toroidal CVTs have input and output disks arranged to be rotatable about a common rotation axis, and a plurality of power rollers interposed between the input and output disks in friction contact therewith via an oil film. Upon operating the toroidal CVTs, the power rollers are pressed between the input and output disks by applying thereto a thrust corresponding to a transmission torque. A shear force of the oil film is caused corresponding to the pressing force applied to the power rollers. Owing to the shear force, the power rollers transmit power between the input and output disks. Each of the power rollers is supported on a trunnion so as to be rotatable about a rotation axis and pivotally moveable about a pivot axis (trunnion axis) perpendicular to the rotation axis of the power roller. Upon the speed change operation of the toroidal CVT, the trunnion is displaced or offset from the non-speed change position where the rotation axis of the power roller is perpendicular to the common rotation axis of the input and output disks, along a direction of the pivot axis of the power roller. With the displacement of the trunnion, the power roller is allowed to pivotally move about the pivot axis due to component of the rotation force of the input disk. This causes change in the contact position of the power roller relative to each of the input and output disks, then causing a variation of the rotation speed ratio between the input disk and the output disk, i.e., a speed ratio. Thus, the speed ratio of the toroidal CVT can be continuously varied.
The power rollers tend to be pushed out from a toroidal cavity formed by contact surfaces of the input and output disks during the operation of the toroidal CVT. In order to avoid the push-out of the power rollers from the toroidal cavity, upper end portions of the trunnion are joined together with those of the adjacent trunnion via an upper link, and lower end portions of the trunnion are joined together with those of the adjacent trunnion via a lower link. The upper and lower end portions of the trunnion are connected with the upper and lower links by means of a combined joint which is constituted of an outer spherical joint and an inner bearing. The combined joint is mounted to each of the end portions of the trunnion and engaged with a trunnion connection hole formed in each of the upper and lower links.
However, the pressing force applied to the power rollers tends to cause elastic deformation of the trunnions and the input and output disks. Due to the elastic deformation, the power rollers are displaced from a predetermined contact position or a predetermined speed ratio position relative to the input and output disks in which a target speed ratio is obtained.
Japanese Patent Application First Publication No. 6-280959 discloses a power roller support structure for holding the power rollers in the predetermined speed ratio position upon the occurrence of the elastic deformation of the trunnions and the input and output disks.
FIG. 8A is an explanatory diagram schematically illustrating the power roller support structure of the related art as described above. As illustrated in FIG. 8A, end portion A of a trunnion, namely, an outer spherical joint of a combined joint, is engaged with trunnion connection hole B of a link. A circumferential surface of end portion A of the trunnion bears against the periphery of trunnion connection hole B when power roller F is pressed between the input and output disks due to the thrust applied to the input disk. Trunnion connection hole B is formed concentrically with center D of curvature of contact surface C of the respective input and output disks. Radius R of trunnion connection hole B is selected such that power roller F is placed in a position approaching close to a common rotation axis of the input and output disks along rotation axis G of power roller F, more specifically, power roller F is located in such a position projecting downwardly as viewed in FIG. 8A.